History of Animals is a collection of observations about animals. Some classifications of animals by Aristotle are better than those used in the 18th Century. In order to classify animals, Aristotle considers the anatomy, reproduction etc. For example, he says that cetaceans are mammals; snails are similar to marine molluscs (like octopuses, squids etc). In Parts of Animals he uses his logic tools to discuss the observations collected in the History of Animals. He compares the anatomy and physiology of parts and organs, which adapt animals to their habitat. Charles Darwin in a 1882 letter said: "Linnaeus and Cuvier have been my two gods, though in very different ways, but they were mere school-boys to old Aristotle."

Charles Darwin -The Origin of Species by Means of Natural Selection

This book is the account of all the reasons why it's logical and sound to assume that animals evolved in time, and that they all descend from common species. The book focuses on the central idea of evolution of the species and tries to prove the soundness of the theory, as opposed to other theories (like creationism). Darwin gives examples of animals, plants, and breeding of domestic animals, to make his points. Darwin had to put a lot of effort to refute primitive theories like creationism, which some people still considered as sound theories. In an ideal world, religion should not interfere with science.

On The Natural Faculties by Galen

Galen uses the experimental method to investigate the physiology of the organs of animals. For example, he investigates on living animals the flow of urine, by tightening the ducts which extend from the kidneys into the bladder, and observing that the bladder is empty and the ducts are filled with urine.